Various applications use multi-wavelength sensors. Currently there are two solutions generally employed for multiple wavelength staring sensors, namely the use of single multi-wavelength focal plane array sensors, or the use of multiple co-aligned single wavelength focal plane array sensors.
Using multiple separate co-aligned single wavelength focal plane array sensors entails independently aligning the individual sensors with high precision, typically using a beamsplitter and alignment targets. As a result, this approach generally uses highly sophisticated mounting systems which will accommodate alignment correction while holding the focal plane array sensors accurately in their relative positions during operation of the system and over the operating temperature range. A further complication with this approach is that the sensors are generally aligned at ambient temperature; however, they will often be cryogenically cooled during operation, which can lead to alignment errors. Using multi-wavelength focal plane array sensors simplifies the alignment of the wavelength bands since the alignment is done during manufacturing of the sensor array itself. For example, the different wavelength detectors may be grown on top of each other on a semiconductor wafer. However, this process increases the cost of the sensor and risk of defects that result from the additional processing and handling steps during manufacture, and reduces the array's performance. For example, defective pixels in one wavelength band are typically correlated with defective pixels in another wavelength band since the band-detectors are grown on top of one another.